Avoid undefined behavior in gzwrite.c.

Adding to NULL is undefined.

gzwrite.c

diff --git a/gzwrite.c b/gzwrite.c
index 13a3700..b5026e5 100644
--- a/gzwrite.c
+++ b/gzwrite.c
@@ -383,7 +383,8 @@
     z_streamp strm;
 
     strm = &(state->strm);
-    if (strm->next_in + strm->avail_in <= state->in + state->size)
+    if (strm->next_in == NULL ||
+        strm->next_in + strm->avail_in <= state->in + state->size)
         return 0;
     (void)gz_comp(state, Z_NO_FLUSH);
     if (strm->avail_in == 0) {